1. Field of the Invention
The present invention relates generally to a signal equalizer for removing distortions from a communications signal and, more particularly, to a signal equalizer employing a feed forward equalizer (FFE) processor, a decision feedback equalizer (DFE) processor and an adaptive weight processor for correcting distortions in an optical communications signal, where the adaptive weight processor employs a correlative error detection system for correlating the corrected signal with a sequence of correlation values and for detecting signal spikes in the correlated signal to properly set weight values applied to weight taps in the FFE processor and the DFE processor.
2. Discussion of the Related Art
Modern high speed digital communications systems transmitting very high data rates, 10-40 Gbps, typically transmit optical signals carrying digital data down a fiber optic cable to transfer the data. The fiber optic cable includes a waveguide core having one index of refraction that is surrounded by a cladding layer having another, lower, index of refraction. Optical signals propagating down the core at a certain angle of incidence are reflected off of the core/cladding transition to be contained therein.
The optical signals propagating through the fiber optic cable are distorted by the waveguide medium. This distortion may cause loss of data when the optical signal is demodulated at the receiver to remove the information therefrom. Optical distortion typically comes from two sources, chromatic dispersion and polarization modulation dispersion. Polarization modulation dispersion occurs because the light in the optical signal having one polarization orientation travels faster than the light in the signal at other polarization orientations. In other words, regardless of whether the polarization of the signal is circular, elliptical, linear, etc., the axis of the polarization rotates so that when the signal is oriented along one particular axis it will travel faster than when it is oriented along another axis.
Chromatic dispersion is related to the frequency of the optical signal, where chromatic dispersion increases the farther the signal propagates and the higher the signal bandwidth. Further, light at higher frequencies propagates faster than light at lower frequencies. Chromatic dispersion occurs because as the optical signal propagates through the fiber optic cable and is reflected off of the transition between the core and cladding layer, some parts of the signal have a different path length than other parts of the signal and thus travel a different distance. Because the optical signal may travel through the fiber cable several thousand miles between a transmitter and a receiver, the distortion may be significant.
Both chromatic dispersion and polarization dispersion cause the digital coded bits in the signal to be at relatively different times (early or late) in the signal than would be expected, thus affecting the ability to recover the bits in the receiver. Particularly, some of the several parts of a bit may be included in the symbol of a previous or next symbol. Thus, the distortion affects the reliability of decoding the bits to remove the information. Because the distortion is linear, the process that distorted the signal can be inverted to provide a corrected signal.
It is known in the art to employ equalizers in the receiver of a fiber optic communications systems for reconstructing the corrupted signal before it is decoded to correct for distortions caused by chromatic and polarization dispersion. Generally, the equalizer restores the waveform of the signal by equalizing the effects of the distortions caused by the fiber optic cable. The equalizer defines a mathematical polynomial that is the inverse of the distortion process caused by the fiber cable. Equalizers known in the art include a finite impulse response (FIR) equalizer, sometimes called a feed forward equalizer (FFE) processor, and an infinite impulse response (IIR) equalizer, sometimes called a decision feedback equalizer (DFE) processor. The DFE processor is different than the FFE processor because it employs a feedback loop. However, as is understood in the art, FFE processors and DFE processors have different advantages and drawbacks.
The optical signal is converted to an RF analog signal prior to being applied to the equalizer. For very high frequency applications, the wavelength of the signal is short enough that it easily fits on small integrated circuit chips. The equalizer samples the RF analog signal at spatial locations along the signal waveform. The signal is multiplied by a weight value applied to a tap at each sample location to provide the distortion correction. Each weighted signal is summed with the weighted signals from the other sample location. Thus, the distortions in the signal are corrected through the weighting process.
The equalizers known in the art are effective for correcting signal distortions in optical communications systems. However, as the processor speeds and data rate speeds increase, it becomes increasingly more difficult to sample the signals by the known techniques to provide the signal reconstruction because the electronic elements, such as analog-to-digital converters, multipliers, summers, etc., would need to be too large, require too much power and would be unable to operate fast enough. For example, to correct a 40 Gbps signal, the signal would need to be sampled at the Nyquist sampling rate or greater, which could be about 120 giga-samples per second. At this sample rate, the sampled signal would have to be delayed, multiplied and summed in each sample stage of the equalizer in about 25 picoseconds. Thus, it is necessary to provide different techniques than are currently known to sample and weight the distorted signals in an equalizer at state of the art data rates.